In U.S. Pat. No. 4,085,644 there is described a Polyphonic Tone Synthesizer in which a master data set is computed and stored in a main register from which it is transferred to note registers of a plurality of tone generators. The master data set defines the amplitudes of equally spaced points along a half cycle of the audio waveform of the musical tones being generated. Each tone generator receives the binary words in the master data set and applies them to a digital-to-analog converter at a rate determined by the fundamental pitch of the respective tones being generated by the Polyphonic Tone Synthesizer.
One of the features of the Polyphonic Tone Synthesizer, as described in the above-identified patent, is that the transfer of successive words from the master data set in the main register to an individual note register in the respective tone generators is synchronized with the transfer of binary words from the note register to the digital-to-analog converter in the respective tone generators. This feature permits the master data set defining the waveform to be recomputed and loaded in the respective tone generators without interrupting the generation of the respective musical notes by the tone generators, thus permitting the waveform of a musical tone to be changed with time without interrupting the resulting musical tone.
There are several criteria for classifying types of electronic keyboard musical instruments. They can be typed as being monophonic or polyphonic depending upon whether or not they are implemented to play one or more than one note at a time in response to actuated keyboard switches. These instruments can be further classified by the manner in which tones at different fundamental pitches are generated. Pitches in organ-like musical instruments are commonly designated by footages following established pipe organ terminology. Thus an 8-foot pitch, also called the unison pitch, will be one for which the keyboard note A.sub.4 will cause a tone to be generated having a fundamental frequency of 440 hz.
Two principal techniques are usually employed to generate tone pitches other than the unison pitch. The first technique is that which is known by the generic term "straight." A pipe organ constructed to use straight tone implementation will have a separate and independent rank (set) of pipes corresponding to each stop (tone control). Thus an 8-foot stop will be associated with its own dedicated set of pipes. A 4-foot stop will also have its own dedicated set of pipes. If both an 8-foot and 4-foot stop are drawn (actuated), then each actuatated keyboard switch will cause two individual pipes to be blown.
The advent of the theatre organ caused another tone combination system to flourish and reach rather advanced and complex forms. The principal tonal contribution of the theatre organ development was the use of what is known by the generic name of "unified" stops. In a unified organ a 4-foot stop is obtained from a parent 8-foot rank by mechanically, or electrically, causing a note to sound one-octave higher than that associated with the actuated keyboard switch. Thus if an 8-foot and 4-foot unified stop combination is selected each keyed note will cause the unison pipe and one in the next higher octave to sound simultaneously. The same general keyboard switching scheme is readily extended to other footages such as the commonly used set of pitches: 16', 4', 21/3', 2', 1 3/5', 1'. In this fashion unification has been used to obtain a fairly large number of stops from a single rank of pipes. Thus one advantage of unification lies in the economics of extending the use of a single rank of pipes to a number of stops.
There are some tonal deficiencies that are inherent with most unified organ designs. First there is the problem of missing notes that frequently perplexes beginning players on unified instruments. If an 8-foot and 4-foot combination is drawn (two actuated stops) and C.sub.4 is keyed then the instrument will simultaneously play C.sub.4 and C.sub.5. If C.sub.4 and C.sub.5 are keyed with the same stop combination, the instrument will sound the notes C.sub.4, C.sub.5, C.sub.6. Only three, rather than four, notes will sound because C.sub.5 corresponds to the 4-foot pitch for the keyed note C.sub.4 and to the 8-foot pitch for the keyed note C.sub.5. If C.sub.5 is played after first playing C.sub.4, the neophyte player is confused because he does not hear the added unison tone for C.sub.5 which is already sounding because of the 4-foot stop. All he hears is the addition of C.sub.6 which is not the fundamental tone for the actuated C.sub.5 note switch. A second tonal characteristic of unified instruments is that unified systems cause the lower footages (higher pitches) to sound much louder than is desireable because there is no separate loudness compensation for the lower footages which in reality are simply "borrowed" or unified from a single unison rank.
A straight organ design will usually employ different harmonic structure for the higher pitches in common tone families. Moreover, the lower footage ranks are generally made increasingly softer with decreased footage so that their combination blends musically with the unison pitch tones.
A system for unifying an electronic organ is described in U.S. Pat. No. 3,697,661 entitled "Multiplexed Pitch Generator For Use In A Keyboard Musical Instrument." This system operates by using a conventional time division multiplex of the keyboard switches. Unification is obtained by inserting keyed positions into later slots in the multiplex time scan after delays depending upon the desired unification pitches.
Digital tone generators such as those described in U.S. Pat. No. 3,515,792 entitled "Digital Organ"; U.S. Pat. No. 3,809,789 entitled "Computer Organ"; and U.S. Pat. No. 4,085,644 entitled "Polyphonic Tone Synthesizer" are generally implemented as straight organs. This choice is made partly for tonal reasons and partly because in such systems it has been found to be far more economical to implement a straight organ design for different stop pitches than to add the extra circuitry that would be required to obtain a unified tonal stop design. In the referenced digital tone generation systems higher pitched (lower footage) stops are obtained by a scheme of harmonic suppression. For example, a 4-foot stop is implemented by using only the even harmonics from the total set of harmonics and suppressing all the odd harmonics. A 22/3-foot stop is implemented by using only the harmonic sequence 3,6,9,12,15, . . . and suppressing all other harmonics. This technique of harmonic suppression produces a good approximation to "proper" organ tonal design in that an independent loudness can be designed for each stop regardless of footage and each stop has its own set of harmonic components which can be independent of any other stop or footage. A negative tonal attribute to the method of using harmonic suppression for lower footage stops is that all the combined stops are phase locked to the unison pitch.
Unified stops are desirable if one wishes to imitate the tonal characteristics of the very popular American theatre organ which is based upon the liberal use of unification. The referenced digital tone generators can be unified in a straightforward manner by using a keyboard system such as that described in the above referenced U.S. Pat. No. 3,697,661 and by adding additional tone generators. The basic digital organs usually have 12 tone generators. An additional set of 12 tone generators are required for each unified pitch. These additional sets of tone generators quickly makes unification prohibitive in cost.
It is an objective of the present invention to obtain unified tonal design in the Polyphonic Tone Synthesizer without increasing the number of tone generators and without increasing the speed of the system timing clocks.